This invention relates to the field of broadband communication signal processing such as in cable access television CATV and more specifically to the field of broadband baluns for use in line amplifiers.
A balun is a component used in radio frequency systems to transfer signals between an unbalanced transmission line and a pair of balanced transmission lines. In a balanced transmission line a pair of lines carry equal current signals that are 180xc2x0 out of phase. Twisted wire pairs are typically used for balanced transmission lines. In an unbalanced transmission line, such as a coaxial cable, the currents in the two conductors are unbalanced with respect to a symmetrical ground plane.
Such baluns are often produced by winding a coil of fine bifiler wire (e.g. #34 or #36 wire) or fine coaxial cable around a small toroidal or rod-shaped core of ferrite. The performance of this form of balun is highly dependent on the position of the wires with respect to each other and with respect to the core, so that additional tuning and rework are usually required. For such wound baluns, insertion loss is typically approximately 0.8 dB at 900 MHz due to losses in the ferrite core and in the fine wire that is used. Excessive insertion loss can result in reduced efficiency and higher distortion levels when the balun is used with push-pull amplifiers
Alternately, baluns may be produced using cylindrical ferrite beads having single axial holes through which bifiler wire or bare coaxial cable is threaded. The length of the wire should be less than approximately xe2x88x92/4 at the highest frequency of operation, and the number of beads should be sufficient to provide adequate low frequency performance. Ferrite beads for such application is described in xe2x80x9cFerrite Beads, Balun and Broadband Corexe2x80x9d, in Power Conversion International, Vol. 6, No. 4, pp. 44-50, July-August 1980. An example of a beaded balun is described in xe2x80x9cTransforming the Balunxe2x80x9d by John S. Belrose in QST, June 1991, pp. 30-33. That citation describes a balun with 50 beads of 73 ferrite (e.g. Amidon No. FB-73-2401) threaded over a 12 inch portion of Teflon dielectric coaxial cable for use from 1.8 to 30 MHz.
A typical application for such baluns is in push-pull operation of a linear amplifier in a broadband network. In such an amplifier, the power of the signal in both lines of a balanced transmission line is amplified. xe2x80x9cBroadband Transformer Design for RF Transistor Power Amplifiersxe2x80x9d by Octavius Pitzalis Jr. and Thoman Couse in ECOM-2989, pp. 207-216, U.S. Army Electronics Command, Fort Monmouth, N.J., July 1986, describes such an amplifier. A first balun converts an unbalanced signal in a coaxial cable into a balanced signal in a balanced transmission line pair, then the balanced signal is amplified by several amplifier stages, and then a second balun converts the balanced signal into an unbalanced signal for transmission through another coaxial cable. In modern wideband GaAs FET linear amplifiers, the frequency response is often limited by the baluns that are used.
U.S. Pat. No. 5,808,518 to McKinzie, III et. al. describes a 4:1 balun for use with signal frequencies between 100 MHz and 10 GHz. In that citation, a circuit board has a window and two narrow elongate cantilever portions of the circuit board extend from a first side of the window. The cantilever portions carry straight printed sections of a U-shaped conductor of the circuit board. Four beads are threaded over each of the cantilever portions, and wires are connected from the free ends of the cantilever portions to the circuit board at a second side of the window to form the balun. U.S. Pat. No. 5,379,006 describes baluns formed by threading beads over coaxial cable for use in ultra wide band (e.g. DC to GHz) transmission. That balun is supported by a circuit board with a window through which a portion of the balun extends onto the other side of the circuit board. U.S. Pat. No. 5,742,205 to Cowen et. al. describes FET CATV line amplifiers that utilize input and output baluns.
Transmission line transformer theory and example designs are described in xe2x80x9cTransmission Line Transformersxe2x80x9d, 1st edition, by Jerry Sevick, Amateur Radio Relay League, Newington Conn., 1987; and in xe2x80x9cBuilding and Using Baluns and Ununsxe2x80x9d by Jerry Sevick, from CQ Communications Inc, Hicksville, N.Y., 1994.
All of the above citations are hereby incorporated herein in whole by reference.
It is an object of the invention to provide a 1:1 balun for high performance broadband applications (i.e. in a frequency range from 45 MHz to 1005 MHz).
It is an object of the invention to provide such a balun with performance that insensitive to manufacturing tolerances in the position of the wires with respect to each other or with respect to the ferrite cores, so that additional tuning and rework is minimized and automated manufacturing of the balun is easily achievable.
It is an object of the invention to provide such a balun in which the input terminals are proximate to the output terminals of the balun for compact circuitry.
It is an object of the invention to provide such a balun that is lightweight and compact.
It is an object of the invention to provide such a balun that can be easily and securely mounted on a circuit board in automated manner.
It is an object of the invention to provide a linear amplifier circuit for push-pull operation in such broadband applications that utilizes such baluns for input and output.
It is an object of the invention to provide such a balun with low insertion power loss (e.g. less than approximately 0.2 dB at 900 MHz);
It is an object of the invention to provide such a balun with low return loss (e.g. approximately 20 dB minimum from 45 to 1005 MHz); and
In the invention, a balun is constructed from a transmission line pair that are bent to form a U-shape including two less curved (straight leg) sections connected at one end by a more curved (bent) section. Single hole ferrite beads are threaded over the less curved sections of the lines. This balun is simple, very compact, and easily adapted for automatic manufacture. Since the input and output of the balun are proximate, the design of circuitry utilizing the balun is simplified. The balun provides high quality signal processing and is useful for high performance broadband push-pull amplifiers in CATV distribution systems especially for signal amplification prior to laser transmission.
The invention includes a high-performance compact push-pull amplifier for use in broadband networks. The compact amplifier includes an input balun for converting an unbalanced signal in a single line into balanced signals in a balanced transmission line pair. A cascade of amplifier stages, in each conductor of the transmission line, amplifies the input signal. An output balun converts the amplified balanced signals in the balanced pair of conductors of the transmission line into an unbalanced signal in a single output line.
The invention also includes a CATV distribution network in which the compact high-performance push-pull amplifier of the invention is used in the head-end and nodes of the network for amplification of the feed to a laser diode transmitter. This provides high quality optical signals in the forward and return directions in the network.
Those skilled in the art can understand the invention and additional objects and advantages of the invention by studying the description of preferred embodiments below with reference to the following drawings that illustrate the features of the appended claims: